1. Field of the Invention
The present invention relates to a process for preparing low-odor and storage-stable monomer-containing polyisocyanurates from isophorone diisocyanate.
2. Discussion of the Background
Polyisocyanurates as polyisocyanate adducts are valuable components for producing high-quality coatings having good mechanical properties and good light and weather resistance. Polyisocyanurates derived from isophorone diisocyanate (IPDI) are also used as raw material for elastomer applications. Here, it can be desirable for the IPDI-based polyisocyanurate, also referred to as IPDI trimer, to be used in monomer containing form.
Polyisocyanurates are typically obtained by the catalytic trimerization of suitable isocyanates. Suitable isocyanates include, for example, aromatic, cycloaliphatic and aliphatic bifunctional and higher-functional polyisocyanates. Suitable catalysts include, for example, tertiary amines (U.S. Pat. No. 3,996,223), alkali metal salts of carboxylic acids (CA 2 113 890; EP 056 159), quaternary ammonium salts (EP 798 299; EP 524 501; U.S. Pat. No. 4,186,255; U.S. Pat. No. 5,258,482; U.S. Pat. No. 4,503,226; U.S. Pat. No. 5,221,743), amino silanes (EP 197 864; U.S. Pat. No. 4,697,014) and quaternary hydroxyalkylammonium salts (EP 017 998; U.S. Pat. No. 4,324,879). Depending on the catalyst, the use of various cocatalysts is also possible, e.g. OH-functional compounds or Mannich bases derived from secondary amines and aldehydes or ketones.
To carry out the trimerization, the polyisocyanates are allowed to react in the presence of the catalyst, if desired with addition of solvents and/or auxiliaries, until the desired conversion has been reached. In this context, the term xe2x80x9cpartial trimerizationxe2x80x9d is typical since the desired conversion is generally significantly below 100%. The reaction is then stopped by deactivation of the catalyst. Deactivation is achieved by addition of a catalyst inhibitor such as p-toluene sulfonic acid, hydrogen chloride or dibutyl phosphate; and unavoidably and often undesirably results in the contamination of the resulting polyisocyanate containing isocyanurate groups.
In the trimerization of isocyanates on an industrial scale, the use of quaternary hydroxyalkylammonium carboxylates as oligomerization catalysts is particularly advantageous. This type of catalyst is thermally labile and allows targeted thermal deactivation, so that it is not necessary to stop the trimerization by addition of potentially quality-reducing inhibitors when the desired conversion has been reached.
Monomer-containing IPDI trimer, which is suitable, for example, for elastomer applications, has an NCO content of at least 25% by weight for viscosity reasons. The polyisocyanurate is prepared by partial trimerization of IPDI in the presence of one or more suitable catalysts. The catalyst must then either be removed completely from the reaction solution, which can be achieved by short-path distillation or thin-film evaporation, or be deactivated because the trimer is not storage-stable in the presence of active catalyst residues. If the NCO content of the IPDI polyisocyanurate obtained is below the desired level, it can easily be adjusted as desired by diluting the solution with monomeric IPDI.
Alkali metal salts of carboxylic acids are not well suited as catalysts for the preparation of monomer containing IPDI trimer since these catalysts can be removed from the reaction products only with difficulty, if at all. With respect to the available amine-containing catalysts, it has been found that the resulting IPDI trimer solutions usually have a distinctly perceptible and undesirable odor, which is sufficiently pronounced to be noticeable and unpleasant in the final application. In industrial practice, the undesirable odor is eliminated by freeing the reaction solution after partial trimerization and catalyst deactivation of excess IPDI, of odor-imparting components and possibly of undesirable catalyst inhibitors. This freeing is generally achieved by short-path distillation or thin-film evaporation. The solid resin that has been freed of monomer is subsequently converted by the addition of fresh IPDI into the desired, low-odor and monomer-containing IPDI polyisocyanurate.
The sequence of partial trimerization/deactivation, monomer removal/purification and subsequent dissolution of the solid resin in the monomer is very complicated. The monomer removal step is particularly time-consuming and costly, and it limits the capacity or and creates a bottleneck in the known processes.
It is an object of the present invention to provide a process for preparing monomer containing polyisocyanurates that avoids the problems associated with conventional processes.
It is another object of the present invention to provide a more economical process for preparing monomer-containing polyisocyanurates.
It is another object of the present invention to provide a process for preparing low-odor monomer-containing polyisocyanurates.
It is another object of the present invention to provide a process for preparing storage-stable monomer-containing polyisocyanurates.
It is another object of the present invention to provide a process for preparing low-odor and storage-stable monomer-containing polyisocyanurates from isophorone diisocyanate.
It is another object of the present invention to provide a process for preparing monomer-containing polyisocyanurates which avoids the monomer removal step.
It is another object of the present invention to provide a more economical process for preparing low-odor and storage-stable monomer-containing polyisocyanurates from isophorone diisocyanate which avoids the need for the monomer removal step.
It is another object of the present invention to provide low-odor and storage-stable monomer-containing polyisocyanurates.
It is another object of the present invention to provide low-odor and storage-stable monomer-containing polyisocyanurates that avoids the need for quality-reducing catalyst inhibitors.
It is another object of the present invention to provide low-odor and storage-stable monomer-containing polyisocyanurates that avoids the need for monomer removal and/or chemical deactivation of the isophorone diisocyanate trimerization catalyst.
The objects of the present invention, and others, may be accomplished with the present invention, the first embodiment of which provides a process, including:
partially trimerizing isophorone diisocyanate in the presence of a catalyst having the formula:
[Rxe2x80x94NX3]⊕Yxe2x8ax96
wherein R and X are butyl groups and Yxe2x88x92 is CH3COOxe2x88x92; or
wherein R is a benzyl group, Yxe2x88x92 is a carboxylate anion having from 4 to 8 carbon atoms and each X is an alkylene group having from 2 to 3 carbon atoms, wherein the three alkylene groups share a common carbon atom and, together with the N atom in the formula, form a tricyclic structure, and wherein at least one alkyene group has at least one OH group in an xcex1 or xcex2 or xcex3 position relative to the N atom,
to obtain a monomer-containing polyisocyanurate mixture.
Another embodiment of the present invention provides a monomer-containing polyisocyanurate mixture, prepared by a process including:
partially trimerizing isophorone diisocyanate in the presence of a catalyst having the formula:
[Rxe2x80x94NX3]⊕Yxe2x8ax96
wherein R and X are butyl groups and Yxe2x88x92 is CH3COOxe2x88x92; or
wherein R is a benzyl group, Yxe2x88x92 is a carboxylate anion having from 4 to 8 carbon atoms and each X is an alkylene group having from 2 to 3 carbon atoms, wherein the three alkylene groups share a common carbon atom and, together with the N atom in the formula, form a tricyclic structure, and wherein at least one alkyene group has at least one OH group in an xcex1 or xcex2 or xcex3 position relative to the N atom.
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description of the preferred embodiments of the invention.
Preferably, the process includes preparing low-odor and storage-stable monomer containing polyisocyanurates from isophorone diisocyanate by partial trimerization over a period of from 30 seconds to 2 hours in the presence of from 0.01 to 2% by weight, based on the weight of the diisocyanate, of a catalyst of the formula:
[Rxe2x80x94NX3]⊕Yxe2x8ax96
where R and X are butyl groups and Yxe2x88x92 is CH3COOxe2x88x92, or R is a benzyl group and Yxe2x88x92 is a carboxylate anion having from 4 to 8 carbon atoms and in this case X is an alkylene group having from 2 to 3 carbon atoms, with the three radicals X together with the quaternary nitrogen forming, via a common carbon atom, a tricyclic structure which has at least one OH group in the xcex1 or xcex2 or xcex3 position relative to the nitrogen, at a temperature of from 0 to 200xc2x0 C.
By use of the present invention, monomer removal and chemical deactivation of the trimerization catalyst can be omitted. It could not have been foreseen that the present invention, which requires, inter alia, the claimed catalysts would result in a such an economical process.
Isocyanates suitable for the trimerization can be prepared by various methods (Annalen der Chemie 562 (1949), p. 75 ff, the entire contents of which are hereby incorporated by reference). A method which has been found particularly useful in industry is phosgenation of organic polyamines to form the corresponding polycarbamic acid chlorides and thermal dissociation of these into organic polyisocyanates and hydrogen chloride. As an alternative, organic polyisocyanates can also be prepared without the use of phosgene, i.e. by phosgene-free processes. According to EP 126 299 (U.S. Pat. No. 4,596,678), EP 126 300 (U.S. Pat. No. 4,596,679) and EP 355 443 (U.S. Pat. No. 5,087,739), the entire contents of each of which being hereby incorporated by reference, (cyclo)aliphatic diisocyanates such as 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate or IPDI) can, for example, be obtained by reaction of the parent (cyclo)aliphatic diamines with urea and alcohols to form (cyclo)aliphatic biscarbamic esters and thermal dissociation of these into the corresponding diisocyanates and alcohols.
As far as the process of the invention for preparing low-odor and storage-stable monomer-containing polyisocyanurates from isophorone diisocyanate is concerned, the synthetic route by means of which the IPDI used has been prepared is not particularly limited. However, it may be pointed out that the preferable amount of catalyst used to achieve a desired NCO content is dependent, inter alia, on the quality of the raw material. Experience has shown that an increasing content of hydrolyzable chlorine compounds in the IPDI makes an increase in the amount of catalyst necessary. The hydrolyzable chlorine apparently tends to have an inhibiting effect on the catalyst.
Preferably, to prepare the tricyclic trimerization catalysts, a two-stage synthetic route can be employed. In the first step, the parent tertiary tricyclic amine is quaternized by means of a benzylating agent. Suitable benzylating agents are, for example, benzyl chloride, benzyl bromide, benzyl iodide, benzyl tosylate or benzyl triflate, while a suitable amine is, for example, 3-hydroxyquinuclidine. The quaternization occurs at from 0xc2x0 C. to 100xc2x0 C., preferably 10xc2x0 C. to 90xc2x0 C., and more preferably 20xc2x0 C. to 80xc2x0 C. and can be carried out in the presence or absence of solvents. These ranges include all values and subranges therebetween, including 5, 15, 25, 35, 45, 55, 65, and 75xc2x0 C. The solvent-based process is generally preferred.
In the second step, the quaternary, tricyclic ammonium salt obtained is converted into the desired catalyst. For this purpose, a basic ion exchange resin (e.g. Amberlyst, Dowex or Sephadex) is activated with aqueous potassium hydroxide or aqueous sodium hydroxide and loaded with the desired carboxylic acid. Examples of suitable carboxylic acids are pivalic acid, hexanoic acid, 2-ethylhexanoic acid, adipic acid and succinic acid. The quaternary ammonium salt is then introduced onto the chromatographic column and eluted. The eluate comprises the desired quaternary ammonium carboxylate. The solvent can be removed by application of vacuum. In the case of the quaternary ammonium halides, the catalysts can also be obtained in very pure form by cation exchange in solution if the silver carboxylates of the specified carboxylic acids are used as reactants. It is also possible to convert the quaternary ammonium salts firstly into the corresponding quaternary ammonium hydroxides by means of ion exchange chromatography and then to convert these into the quaternary ammonium carboxylates by reaction with the desired carboxylic acid, possibly with the removal of the water liberated.
The preparation according to the invention of the low odor and storage-stable monomer-containing polyisocyanurates from isophorone diisocyanate by partial trimerization can be carried out continuously (tube reactor or reactor cascade) or batchwise. The catalyst is preferably used in a low concentration in the range from 0.01 to 2% by weight. The precise amount can easily be determined experimentally and depends on the catalyst, on the intended conversion, on the quality of the IPDI used and on the way in which the process is carried out.
Preferably, the partial trimerization is carried out over a period of from 30 seconds to 2 hours, more preferably from 1 minute to 1.5 hours, and most preferably from 5 minutes to one hour. These ranges include all values and subranges therebetween, including 45 seconds, and 1.5, 7, 9, 10, 25, 35, 45, 55, 65, 75, 85, 95, 100, and 115 minutes.
The term xe2x80x9cmonomer-containing polyisocyanurate mixturexe2x80x9d preferably means a mixture containing at least one or more isocyanurates, polyisocyanurates, or both, and monomeric IPDI. In addition to monomeric IPDI, the product includes compounds which have one or more isocyanurate rings. Compounds having a uretdione structure may also be present in small amounts as by-products. Compounds of this type are described in the literature.
The catalyst is preferably used in an amount of from 0.01 to 2% by weight, more preferably 0.04 to 1% by weight, and most preferably 0.09 to 0.8% by weight, based on the weight of the diisocyanate. These ranges include all values and subranges therebetween, including 0.02, 0.03, 0.05, 0.06, 0.07, 0.2, 0.5, 0.7, 1.1, 1.4, and 1.6% by weight, based on the weight of the isophorone diisocyanate used. Preferably, the process of the invention is carried out at temperatures in the range from 0xc2x0 C. to 200xc2x0 C., more preferably from 20xc2x0 C. to 180xc2x0 C., and most preferably from 40xc2x0 C. to 160xc2x0 C. These ranges include all values and subranges therebetween, including 10, 30, 50, 80, 120, 140, 150, 170 and 190xc2x0 C. The process may be carried out either batchwise or continuously. The batch process is preferred.
The batch process is preferably carried out in a stirred reactor. Here, the mixture of isophorone diisocyanate and catalyst is usually placed in the reactor at room temperature. Preferably, the temperature of the reaction mixture is subsequently increased to from 40 to 140xc2x0 C. and more preferably to from 55 to 100xc2x0 C., so as to initiate the trimerization. These ranges include all values and subranges therebetween, including 10, 30, 50, 80, and 120xc2x0 C. As an alternative, the catalyst can also be introduced after the IPDI has reached the temperature necessary for the reaction. However, this variant is not preferred. The trimerization is exothermic. The catalyst can be used in pure form, but it is also possible to dissolve the catalyst in a suitable solvent and to introduce it in this form.
The continuous trimerization is preferably carried out in a reaction loop with continuous, uniform metered addition of IPDI and the catalyst at from 40 to 180xc2x0 C., more preferably from 60 to 160xc2x0 C., and most preferably from 80 to 140xc2x0 C. and preferably over a period of from 30 seconds to 10 minutes, more preferably from 40 seconds to 7 minutes, and most preferably from 55 seconds to 6 minutes, which ranges include all values and subranges therebetween. A reaction loop having a small diameter leads to high flow velocities and consequently to good mixing. It is also preferable to heat the IPDI/catalyst mixture to from about 50 to 60xc2x0 C. before introduction into the reaction loop. For more precise metering and optimal mixing of the catalyst, it is also preferable to dissolve the catalyst in a suitable solvent. Suitable solvents are in principle all those in which the catalyst has a good solubility, e.g. water, low molecular alcohols such as methanol or low molecular weight organic acids such as acetic acid or hexanoic acid. Mixtures are possible.
The continuous trimerization can also be carried out in a reactor cascade. A combination of a reactor cascade and a tube reactor is also suitable.
Preferably, the temperature profile of the process of the invention should be such that the reaction solution reaches a temperature of from 150 to 180xc2x0 C., and more preferably at least from 140 to 160xc2x0 C., which ranges include all values and subranges therebetween, including 152, 155, 161, 168, 174 and 178xc2x0 C. In this way it can be ensured that the product prepared according to the invention meets the criterion of storage stability and thus does not gel during prolonged storage.
The low-odor and storage-stable monomer-containing polyisocyanurates prepared according to the invention from isophorone diisocyanate have an NCO content of from 25 to 34% by weight. This range includes all values and subranges therebetween, including 26, 27, 28, 29, 30, 31, 32, and 33% by weight. They are useful intermediates for polyurethane coatings and elastomer applications. In these applications, they may preferably be used in a form which has been blocked with blocking agents. Suitable blocking agents are, for example, lactams such as 6-caprolactam, oximes such as methyl ethyl ketoxime or butanone oxime, triazoles such as 1H-1,2,4-triazole, readily enolizable compounds such as ethyl acetoacetate or acetylacetone or else malonic acid derivatives such as diesters of malonic acid.